Lithium-ion batteries are the fastest-growing secondary batteries after  nickel-cadmium and nickel-hydrogen batteries. It has become an indispensable  part of people's life. However, lithium-ion batteries are not perfect, and the  biggest problem is the stability of their charge-discharge cycles.

　　This problem plagues engineers in the lithium battery industry. Even though  SES Power has nearly 20 years of lithium battery customization services, we are  still plagued by this problem. To this end, we use high-quality raw materials  and batteries from excellent manufacturers, such as 12V100Ah, 24V100Ah,  36V100Ah, 48V100Ah using EVE, CATL, BYD square aluminum lithium iron phosphate  batteries, home energy storage 3KW, 5KW systems, racks energy storage systems  and other products. But that also means a relatively high price, and it does  make sense that performance dictates the price.

　　The reason for this annoying problem is briefly analyzed by SES Power  below. First, let's start with the principle.

　　Lithium-ion batteries have different intercalation energies when  intercalation reactions occur between the two electrodes, and in order to obtain  the best performance of the battery, the capacity ratio of the two host  electrodes should maintain a balanced value.

　　In lithium-ion batteries, the capacity balance is expressed as the mass  ratio of the positive electrode to the negative electrode,

　　That is: γ=m+/m-=ΔxC-/ΔyC+

　　In the above formula, C refers to the theoretical coulombic capacity of the  electrode, and Δx and Δy refer to the stoichiometric number of lithium ions  embedded in the negative electrode and the positive electrode, respectively. It  can be seen from the above formula that the required mass ratio of the two poles  depends on the corresponding Coulomb capacity of the two poles and the number of  their respective reversible lithium ions.

　　Generally speaking, a smaller mass ratio leads to incomplete utilization of  the negative electrode material; a larger mass ratio may cause a safety hazard  due to the overcharge of the negative electrode. In short, at the optimized mass  ratio, the battery performance is the best.

　　For an ideal Li-ion battery system, the capacity balance does not change  during its cycle, and the initial capacity in each cycle is a certain value, but  the actual situation is much more complicated. Any side reaction that can  generate or consume lithium ions or electrons may lead to changes in the battery  capacity balance. Once the capacity balance state of the battery is changed, the  change is irreversible and can be accumulated through multiple cycles, which has  a negative impact on battery performance. Serious impact.

　　In lithium-ion batteries, in addition to the redox reactions that occur  when lithium ions are deintercalated, there are also a large number of side  reactions, such as electrolyte decomposition, active material dissolution, and  metallic lithium deposition.

　　A: Overcharge

　　a1. Overcharge reaction of graphite negative electrode:

　　When the battery is overcharged, lithium ions are easily reduced and  deposited on the surface of the negative electrode:

　　The deposited lithium coats the negative electrode surface, blocking the  intercalation of lithium. This results in reduced discharge efficiency and  capacity loss. During fast charging, the current density is too large, the  negative electrode is severely polarized, and the deposition of lithium will be  more obvious.

　　a2. Positive electrode overcharge reaction

　　When the ratio of positive electrode active material to negative electrode  active material is too low, positive electrode overcharge is likely to occur.  The capacity loss caused by overcharge of the positive electrode is mainly due  to the generation of electrochemically inert substances (such as Co3O4, Mn2O3,  etc.), which destroy the capacity balance between the electrodes, and the  capacity loss is irreversible.

　　(1) LiyCoO2

　　LiyCoO2→(1-y)/3[Co3O4+O2(g)]+yLiCoO2 y<0.4

　　At the same time, the oxygen generated by the decomposition of the positive  electrode material in the sealed lithium-ion battery accumulates at the same  time because there is no recombination reaction (such as the generation of H2O)  and the flammable gas generated by the decomposition of the electrolyte, and the  consequences will be unimaginable.

　　(2) λ-MnO2

　　The lithium-manganese reaction occurs when the lithium-manganese oxide is  completely delithiated: λ-MnO2→Mn2O3+O2(g)

　　a3. The electrolyte is oxidized when overcharged

　　When the pressure is higher than 4.5V, the electrolyte will oxidize to  generate insolubles (such as Li2Co3) and gas. These insolubles will block the  micropores of the electrode, which will eventually hinder the migration of  lithium ions, resulting in capacity loss.

　　A small amount of electrolyte is consumed each time it is charged, so more  electrolyte is needed when the battery is assembled. For a constant container,  this means that a smaller amount of active substance is loaded, which results in  a decrease in the initial capacity. In addition, if a solid product is produced,  a passivation film will be formed on the surface of the electrode, which will  increase the polarization of the battery and reduce the output voltage of the  battery.

　　B: The water content is too high

　　Excessive water content in the electrolyte will generate LiOH(s) and Li2O  deposits, which are not conducive to lithium ion intercalation, resulting in  irreversible capacity loss:

　　H2O+e→OH-+1/2H2

　　OH-+Li+→LiOH(s)

　　LiOH+Li++e-→Li2O(s)+1/2H2

　　The generated LiOH(s) is deposited on the electrode surface to form a  surface film with high resistance, which hinders Li+ intercalation into the  graphite electrode, resulting in irreversible capacity loss. A small amount of  water (100-300×10-6) in the solvent has no effect on the performance of the  graphite electrode.

　　C: Self-discharge

　　Self-discharge refers to the phenomenon that the battery loses its capacity  naturally when it is not in use. There are two types of capacity loss caused by  self-discharge of lithium-ion batteries: reversible capacity loss and  irreversible capacity loss.

　　Reversible capacity loss means that the lost capacity can be recovered  during charging, while irreversible capacity loss is the opposite, and this part  of the capacity loss cannot be recovered during charging. The negative electrode  active material may interact with the electrolyte to cause self-discharge and  cause irreversible capacity loss.

　　During the manufacturing process of the cell, these factors will affect the  self-discharge performance: the manufacturing process of the positive electrode  material, the manufacturing process of the battery, the properties of the  electrolyte, temperature, and time. The self-discharge rate is mainly controlled  by the solvent oxidation rate, so the stability of the solvent affects the  storage life of the battery.

　　If the negative electrode is in a fully charged state and the positive  electrode self-discharges, the capacity balance in the battery is disrupted,  resulting in permanent capacity loss.

　　During prolonged or frequent self-discharge, lithium may deposit on the  carbon, increasing the capacity imbalance between the electrodes. So why does  SES Power suggest that customers need to keep the battery at a certain capacity  (usually 30% of the remaining capacity) when the battery is not used for a long  time, charge and discharge the lithium battery every 6 months, etc. Especially  high-rate lithium batteries and lithium iron phosphate batteries that can be  used normally at -40 degrees Celsius, these special lithium batteries should pay  more attention to protecting their performance.